Force sensors are known, and there are multiple methods of fabricating these types of sensors. In one method, sensors utilize bonded strain gauges adhered to a flexing substrate within a load path. For example, a simply supported steel beam that is used integral to a load path can have a strain gauge mounted on the beam. The strain gauge is incorporated in a Wheatstone Bridge Circuit configuration and includes an excitation voltage. The circuit is designed to be at balance before deflection (i.e., no load) and the circuit will have a resistance at zero load. During loading, the beam will deflect and the strain gauge will produce a resistance change. This resistance change is a signal that can be converted into a force value imposed on the beam using a signal conditioner. Depending on the type of configuration (e.g., a quarter bridge, a half bridge, a full bridge), the signal will vary and require calibration to obtain the actual force imposed.
Some strain gauges incorporate a thin plastic film with a bonded NiCr (nickel-chromium or nichrome) wire path embedded on the film. When the film is bonded to the beam and the beam is deflected, the NiCr wire will also be subjected to bending causing a deformation of the wire. The deformation of the wire will cause the above mentioned change in electrical resistance.
The flexing substrate must be configured to elastically deform in an elastic region. In the event that the substrate is subjected to permanent deformation, the sensing wire of the strain gauge will be constrained in the deformed state. This will result in inaccurate subsequent readings of the sensor.
Solder connections are typically utilized in a strain gauge circuit, with the wire path of the strain gauge terminating at a pair of solder pads. Other connections are also used, such as laser welding, mechanical forcing of wires to the contact pads, etc.
The solder connections are subject to possible failures if the connections are made in areas of high strain. Such a strain can cause high levels of deformation causing the solder connections to fatigue. Depending on the level of strain, this fatigue can cause failure of the solder pad resulting in a loss of electrical signal rendering the sensor unusable.
If alternate sensors are used, e.g., those fabricated using vapor deposition of brittle materials, this phenomena can become more problematic. Sensor fabricated using vapor deposition include depositing several layers of media to create the sensor. Typically, the first layer consists of a thin layer of glass deposited along a surface that will incorporate the sensing wire. The sensing wire is first deposited along the substrate as a full NiCr covering. A laser then etches away the NiCr until the desired wire path is created having a plurality of solder pads forming a sensing element as described above with respect to the bonded strain gauge. Finally, a covering layer is used to prevent moisture ingress preventing shorts of the wire trace. The covering layer may be a cured epoxy or an RTV sealant (e.g., room temperature vulcanization silicone), or a vapor deposited glass with a region of glass etched away to gain access to the solder pads. This allows for the soldering of the wires or a flex cable to the sensor.
The configurations described above suffer from problems. One problem is the ability to load the substrate in an instrument. When utilizing glass along the substrate, the glass can crack when loaded. Another problem is premature failing due to large strains on the solder pads.